Definición de in-group en inglés:

sustantivo

This heightened awareness becomes personally relevant when one commits to identifying with the ethnic in-group.

I think there's another side to it as well, seen from the other side of the fence: incoming slang is a sort of secret language, expressing exquisitely shaded meanings that are shared among the in-group but are baffling to outsiders.

Such dialects, almost by definition, are media of intimacy: ‘mother tongues’ used in the family and in-groups.